Solar cells based on colloidal quantum dots (QDs) could provide an inexpensive, solution-processed alternative to traditional silicon and thin-film photovoltaics. The highest-performing QD photovoltaic (QDPV) devices today employ a planar heterojunction between a wide-bandgap n-type metal oxide (ZnO or TiO2) layer and a film of p-type lead sulfide (PbS) QDs. Despite rapid advances in recent years, however, planar devices remain limited by a fundamental trade-off between light absorption and charge collection. Our work shows that vertically-aligned ZnO nanowires can decouple absorption from collection: The nanowires penetrate into the QD film and serve as highly-conductive channels for extracting photogenerated electrons from deep within the film, boosting the output current by 50% and the cell efficiency by 35% over planar devices. The demonstrated solar power conversion efficiency of 4.9% is among the highest ever reported for a ZnO-based QDPV.

The primary contribution of this work is the integration of solution-grown 1-D nanostructures with PbS QDs to form the first ZnO/PbS bulk heterojunction QD solar cell. We show that nanostructured architectures can substantially improve QDPV performance, and that a simple, low-temperature, bottom-up growth process can produce nanowire alignment and solar cell performance matching that of top-down synthetic processes, with the added advantage of compatibility with glass and flexible plastic substrates. This study of scalable bottom-up processing of ZnO nanowire-based QD solar cells suggests that 1-D nanostructures may be the key to enhancing the efficiency and the economic viability of quantum dot photovoltaics.